The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Exhaust gas aftertreatment devices clean combustion gases after they have left the combustion chamber of an internal combustion engine driving the motor vehicle, using a mechanical, catalytic or chemical method, in order to comply with the legal pollutant limits.
Diesel engines and modern lean-mix petrol engines work in lean mode, i.e. with an oxygen surplus (λ>1). Conventional three-way catalysts cannot therefore be used. The oxidation of CO (carbon monoxide) and CmHn (incompletely burned hydrocarbons) is still possible with oxygen surplus, in a similar manner to a conventional three-way catalyst, but the NOx (nitrous oxide) must be temporarily stored. The catalytic reduction thereof takes place cyclically with a stoichiometric to rich exhaust gas mixture. Therefore, catalysts with additional chemical elements are required which allow storage of NOx, known as NOx storage catalysts.
In order to achieve this temporary storage of nitrous oxides in the NOx storage catalyst, a noble metal catalyst such as platinum, and an NOx storage component which is usually an earth alkali metal such as barium, are applied to suitable carriers. In the lean, i.e. oxygen-rich atmosphere, the nitrous oxides are oxidated under the effect of the noble metal catalyst, absorbed in the catalyst forming nitrates such as for example barium nitrate, and thus removed from the exhaust gas flow. Regular brief “richening” of the exhaust gas causes these reactions to proceed in the opposite direction, whereby the NOx molecules are returned to the exhaust gas flow and the reducing components present in the rich atmosphere, such as CmHn and/or CO, are further reduced.
When the absorption capacity of the NOx storage catalyst is exhausted, the engine electronics set a rich, substoichiometric, reducing exhaust gas mixture for a few seconds. In this brief regeneration mode, the NOx temporarily stored in the catalyst is reduced to oxygen and hence the NOx storage catalyst is prepared for the next storage cycle. This procedure makes it possible to minimize the pollutant emissions from internal combustion engines operated with an air surplus, and observe pollutant limits.
Enrichment for performing such a regeneration mode of the NOx storage catalyst can be achieved by late fuel injection, by changing the ratio of the fuel quantity on main injection to the fuel quantity on post injection, by means of intake air throttling, by increased exhaust gas recirculation rates, or by other measures.
The duration and frequency of the regeneration modes are determined by the engine control unit as a function of the stored nitrous oxide quantity, exhaust gas temperature, exhaust gas mass flow and other parameters.
DE 10 2015 208 093 A1 discloses an arrangement for exhaust gas aftertreatment in an exhaust system, wherein the arrangement comprises an NOx storage catalyst, a three-way catalyst and, arranged upstream thereof, an SCR catalyst for selective catalytic reduction of nitrous oxides contained in the exhaust gas supplied to the SCR catalyst, wherein an oxygen storage capacity component (OSC) is provided in the SCR catalyst which stores oxygen under operating conditions with a lean exhaust gas mixture, and in operating phases with a rich exhaust gas mixture provides oxygen to support the reduction of nitrous oxides at the SCR catalyst.
However, during the regeneration mode, there may not be sufficient oxygen present to convert all breakthrough hydrocarbons (HC) and/or carbon monoxide (CO) leaving the NOx storage catalyst.
The issue of regenerating an exhaust gas aftertreatment device in which breakthrough hydrocarbons and/or carbon monoxide are also converted is addressed by the present disclosure.